1. Field of the Invention
The present invention relates to a radio communication system and a radio communication device that perform data transmissions while varying the transmission mode composed of a modulation system and a coding ratio and so forth.
2. Description of the Related Art
The adaptive modulation & coding ratio communication system has been developed which varies the coding ratio of an error correction code and the modulation system (multi-valued modulation factor) in accordance with the quality of a propagation path. The system provides a user having a good quality of a propagation path with a high-speed data communication while sacrificing the noise durability, and provides a user having a poor quality of a propagation path with a low-speed data communication while taking full account of the noise durability.
The communication system using such adaptive modulation has been introduced in the radio communication system. As an example, the EDGE (Enhanced Data GSM Environment), CDMA 2000, HDR (High Data Rate) system and so forth can be quoted. Also in the W-CDMA (Wideband Code Division Multiple Access) is additionally adapted the same system (HSDPA: High Speed Downlink Packet Access). This system implements the adaptive modulation & coding ratio according to the basic procedure as follows.    1. a terminal measures the reception quality of a signal transmitted from a base station,    2. the terminal notifies the base station of the modulation system & coding ratio (hereunder, referred to as ‘mode request message’) that is regarded as the optimum from the measurement result of the reception quality,    3. based on the mode request message transmitted from the terminal and the state of the resources at the base station, the base station determines the modulation system & coding ratio (generically called transmission mode) that is allocated in practice, and transmits the parameters (transmission parameters) of the determined transmission mode to the terminal,    4. the base station transmits user data based on the determined transmission parameters,    5. the terminal receives the transmission parameters, and carries out the data reception processing based on the transmission parameters, and    6. when detecting errors in the reception data, the terminal replies a retransmission request to the base station, and when receiving data correctly, the terminal replies a new data transmission request to the base station.    7. the system periodically repeats the above steps 1 through 6.
FIG. 10 and FIG. 11 illustrate this processing procedure. FIG. 10 illustrates the relation between the down control channel that notifies the terminal of the transmission parameters relating to the down data transmission from the base station, the down data channel that transmits the user data from the base station, and the up control channel that transmits the transmission parameter request from the terminal. This drawing shows an example in which the above steps 1 through 6 are carried out within a frame cycle. FIG. 11 illustrates the signal transmission & reception sequence between the terminal and the base station using the above channels, which corresponds to FIG. 10.
The base station varies the data transmission rate in accordance with the reception state (reception quality) of the user terminal during transmitting the down data, and transmits the data to the user terminal more efficiently. Further, taking full account of the system efficiency, the base station allocates a predetermined radio resource for the data transmission to the user terminal having a comparably good reception quality against the long-term average reception quality.
However in general data transmissions, it is necessary to transmit data from the terminal to the base station as a reply for the data that the user received, other than the data transmitted from the base station to the terminal. Although the reception quality is good at the terminal, when there are deficiencies in the data transmission capability, resulting from an insufficiency of the up transmission power and a quality degradation of the up link and so forth, there can be a case that the radio resource for the data transmission allocated to the terminal becomes useless unavoidably. Therefore, it is desirable that the base station carries out the down data transmission in consideration for the current data transmission capability of the terminal.
The present invention has been made in view of these circumstances, and provides a radio communication system and a radio communication device capable of determining a transmission mode in consideration for not only the reception quality of the terminal but also the deficiency in the data transmission capability thereof.